The majority of continental arc volcanoes go through decades or centuries of inactivity, thus, communities become inured to
their threat. Here we demonstrate a method to quantify hazard from sporadically active volcanoes and to develop probabilistic
eruption forecasts. We compiled an eruption-event record for the last c. 9,500 years at Mt Taranaki, New Zealand through detailed
radiocarbon dating of recent deposits and a sediment core from a nearby lake. This is the highest-precision record ever collected
from the volcano, but it still probably underestimates the frequency of eruptions, which will only be better approximated
by adding data from more sediment core sites in different tephra-dispersal directions. A mixture of Weibull distributions
provided the best fit to the inter-event period data for the 123 events. Depending on which date is accepted for the last
event, the mixture-of-Weibulls model probability is at least 0.37–0.48 for a new eruption from Mt Taranaki in the next 50 years.
A polymodal distribution of inter-event periods indicates that a range of nested processes control eruption recurrence at
this type of arc volcano. These could possibly be related by further statistical analysis to intrinsic factors such as step-wise
processes of magma rise, assembly and storage. 相似文献
Tianchi volcano in Changbaishan area is located at the border between China and Democratic People's Republic of Korea, and is one of the most dangerous volcanoes in China. It has experienced several explosive eruptions in late Pleistocene and Holocene, i.e. 50000aBP eruption, 946 AD eruption, 1668 AD eruption, 1702 AD eruption, 1903 AD eruption. Especially, the 946 AD eruption(also known as "Millennium eruption")of this volcano is considered to be one of the largest volcanic eruptions in the world in the past 2000a. The eruption history and strata sequence of Tianchi volcano have long been the focus of attention. The stratigraphic unit division of fallout deposits in the past millennium is controversial, especially for the heterogeneous trachytic pumices(erupted from the Yuanchi stage)above the off-white pumices(erupted from the Chifeng stage).
In this paper, through the detailed field exploration and strata comparation, it was found that there was no depositional interval between the two stage eruptions, or the interval was not long, and thus, it is believed that two stages of fallout pumice should be classified into the Millennium eruption. The off-white fallout pumices in Chifeng stage are relatively homogeneous, with angular shape, normal grading and good sorting. The median size(MdΦ)and the sorting coefficient(σΦ)of Chifeng pumice are in the range of -4.25~-1.3 and 0.93~1.53, respectively. The eruption of Yuanchi stage is in pulsing pattern, and the strata show interbedding of rich khaki pumice layer and rich black pumice layer. The pumices with angular shape show inconspicuous grain grading and good sorting. The median size(MdΦ)and the sorting coefficient(σΦ)of Yuanchi pumice are in the range of -2.55~-0.6 and 1~1.68, respectively. Both the granularities of the pumice particles from two stages are normally distributed and fall into the air-fall field in the median diameter versus sorting diagram. The pumices from 50000aBP and pyroclastic flow of Millennium eruption were also shown in the diagram.
Phenocrysts in pumices are mainly feldspar and pyroxene, but the phenocrysts with obvious resorbed characteristic in Yuanchi black pumice are bigger, and the phenocryst contents are a little higher than those in others. Feldspar content in off-white pumice in Chifeng stage was 0.24%~1.77%, that in khaki pumice in Yuanchi stage was 0.2%~7.5%, and that in black pumice in Yuanchi stage was 3.02%~8.0%. The phenocrysts in Chifeng pumice are broken, which represents more violent explosion. The vesicles inside the pumice also reflect the intensity of the eruption. The Chifeng pumices have large, continuous vesicles and thin vesicle walls. The Yuanchi khaki pumices have continuous vesicles but thicker vesicle wall than the Chifeng pumices. The vesicularity is the lowest and the vesicle walls are the thickest in the black pumices in Yuanchi stage, indicating the eruption strength become weaker from Chifeng stage to Yuanchi stage.
The Chifeng pumices with SiO2 content of 69.12~72.71wt%, K2O content of 4.33~4.52wt%, Na2O content of 5.26~5.39wt%, Al2O3 content of 10.32~11.99wt%, CaO content of 0.29~0.95wt%, MgO content of 0.11~0.51wt%, TiO2 content of 0.23~0.43wt% are comendite in composition. The pumices from 50000aBP eruption are comendite in composition, and their SiO2 content(65.56~68.28wt%)is slightly lower than Chifeng pumices. The Yuanchi khaki pumices with SiO2 content of 62.14~63.29wt%, K2O content of 5.35~5.7wt%, Na2O content of 5.35~5.62wt%, Al2O3 content of 15.00~15.59wt%, CaO content of 1.06~1.61wt%, MgO content of 0.25~0.57wt%, TiO2 content of 0.4~0.64wt% belong to trachyte in composition, and are close to the composition of the black pumices on the Tianwen Peak. The Yuanchi black pumices are also trachyte in composition, but have obviously lower SiO2(59.51~60.59wt%), K2O(4.39~4.84wt%), and Na2O(4.94~5.08wt%)content, and higher Al2O3(15.81~16.42wt%), CaO(2.78~3.66wt%), MgO(1.43~1.9wt%), TiO2(1.04~1.4wt%)content than the khaki pumices.
The above results show that the eruptive intensity of the Yuanchi stage is weaker than that of the Chifeng stage and the several magmatic compositions of pumices from the Millennium eruption reveal a complex magma system under the Tianchi volcano. The magma layers with different compositions may exist in the magma chamber contemporaneously. At Chifeng stage, only the upper comendite magma erupted, but the magma below erupted in the pulsing pattern at the Yuanchi stage. 相似文献
Proximal (<3 km) deposits from episodes II and III of the 60-h-long Novarupta 1912 eruption exhibit a very complex stratigraphy, the result of at least four transport regimes and diverse depositional mechanisms. They contrast with the relatively simple stratigraphy (and inferred emplacement mechanisms) for the previously documented, better known, medial–distal fall deposits and the Valley of Ten Thousand Smokes ignimbrite. The proximal products include alternations and mixtures of both locally and regionally dispersed fall ejecta, and numerous thin complex deposits of pyroclastic density currents (PDCs) with no regional analogs. The locally dispersed component of the fall deposits forms sector-confined wedges of material whose thicknesses halve radially from and concentrically about the vent over distances of 100–300 m (cf. several kilometers for the medial–distal fall deposits). This locally dispersed fall material (and many of the associated PDC deposits) is rich in andesitic and banded pumices and richer in shallow-derived wall-rock lithics in comparison with the coeval medial fall units of almost entirely dacitic composition. There are no marked contrasts in grain size in the near-vent deposits, however, between locally and widely dispersed beds, and all samples of the proximal fall deposits plot as a simple continuation of grain size trends for medial–distal samples. Associated PDC deposits form a spectrum of facies from fines-poor, avalanched beds through thin-bedded, landscape-mantling beds to channelized lobes of pumice-block-rich ignimbrite. The origins of the Novarupta near-vent deposits are considered within a spectrum of four transport regimes: (1) sustained buoyant plume, (2) fountaining with co-current flow, (3) fountaining with counter-current flow, and (4) direct lateral ejection. The Novarupta deposits suggest a model where buoyant, stable, regime-1 plumes characterized most of episodes II and III, but were accompanied by transient and variable partitioning of clasts into the other three regimes. Only one short period of vent blockage and cessation of the Plinian plume occurred, separating episodes II and III, which was followed by a single PDC interpreted as an overpressured "blast" involving direct lateral ejection. In contrast, regimes 2 and 3 were reflected by spasmodic sedimentation from the margins of the jet and perhaps lower plume, which were being strongly affected by short-lived instabilities. These instabilities in turn are inferred to be associated with heterogeneities in the mixture of gas and pyroclasts emerging from the vent. Of the parameters that control explosive eruptive behavior, only such sudden and asymmetrical changes in the particle concentration could operate on time scales sufficiently short to explain the rapid changes in the proximal 1912 products.Editorial responsibility: R. Cioni 相似文献
Thermochemical plumes form at the base of the lower mantle as a consequence of heat flow from the outer core and the presence of local chemical doping that decreases the melting temperature. Theoretical and experimental modelling of thermochemical plumes show that the diameter of a plume conduit remains practically constant during plume ascent. However, when the top of a plume reaches a refractory layer, whose melting temperature is higher than the melt temperature in the plume conduit, a mushroom-shaped plume head develops. Main parameters (melt viscosity, ascent time, ascent velocity, temperature differences in the plume conduit, and thermal power) are presented for a thermochemical plume ascending from the core–mantle boundary. In addition, the following relationships are developed: the pressure distribution in the plume conduit during the ascent of a plume, conditions for eruption-conduit formation, the effect of the P–T conditions and controls on the shape and size of a plume top, heat transfer between a thermochemical plume and the lithosphere (when the plume reaches the bottom of a refractory layer in the lithosphere), and eruption volume versus the time interval t1 between plume formation and eruption. These relationships are used to determine thermal power and time t1 for the Tunguska syneclise and the Siberian traps as a whole.
The Siberian and other trap provinces are characterized by giant volumes of lavas and sills formed a very short time period. Data permit a model for superplumes with three stages of formation: early (variable picrites and alkali basalts), main (tholeiite plateau basalts), and final (ultrabasic and alkaline lavas and intrusions). These stages reflect the evolution of a superplume from the ascent of one or several independent plumes, through the formation of thick lenses of mantle melts underplating the lithosphere and, finally, intrusion and extrusion of differentiated mantle melts. Synchronous syenite–granite intrusions and bimodal volcanism abundant in the margins of the Siberian traps are the result of melting of the lower crust at depths of 65–70 km under the effect of plume melts. 相似文献
Numerous tephra dispersion and sedimentation models rely on some abstraction of the volcanic plume to simplify forecasts of
tephra accumulation as a function of the distance from the volcano. Here we present solutions to the commonly used advection–dispersion
equation using a variety of source shapes: a point, horizontal and vertical lines, and a circular disk. These may be related
to some volcanic plume structure, such as a strong plume (vertical line), umbrella cloud (circular disk), or co-ignimbrite
plume (horizontal line), or can be used to build a more complex plume structure such as a series of circular disks to represent
a buoyant weak plume. Basing parameters upon eruption data, we find that depositions for the horizontal source shapes are
very similar but differ from the vertical line source deposition. We also compare the deposition from a series of stacked
circular disk sources of increasing radius above the volcanic vent with that from a vertical line source. 相似文献